This question is based on a ruling by the European court of human rights, that upheld the right of Germany to have a law against incestuous relationships this week. Here's a comment by the guardian, which raises some good points: http://www.guardian.co.uk/commentisf...against-incest
I think the greatest problem is the "we want to avoid hereditary diseases" point, which the comment makes a good point out of. Sure, it's more probable that a recessive hereditary genetic disease will come through if siblings have children. On the other hand, what is the difference between that and someone with a known family history of depression or other mental or physical impairments procreating?
Also, for the sake of this thread I will have to ask for no more than one redneck joke per commentator :3